1. Field of the Invention
The invention relates to an X-ray mask employed in fabrication of a semiconductor device, and more particularly to such an X-ray mask including an X-ray absorber composed of an alloy having low stress. The invention relates further to a method of fabricating such an X-ray mask, and still further to a method of fabricating a semiconductor device through the use of such an X-ray mask.
2. Description of the Related Art
As a semiconductor device has been integrated highly and highly, there X-ray lithography has been employed for formation of a minute pattern. The X-ray lithography is characterized by features that X-ray has a short wavelength, and that it is relatively easy to have a great depth of focus. The X-ray lithography is suitable particularly for making a small pattern having a length of 10 nm or smaller, or a pattern having a great aspect ratio. The X-ray lithography is employed also in fabrication of a liquid crystal display panel, a charge coupled device (CCD), a thin film magnetic head, and a micro-machine as well as LSI such as memory and logic.
In the X-ray lithography, a resist film is deposited on a wafer, and then, X-ray is radiated onto the resist film through an X-ray mask to thereby make a pattern. Specifically, an X-ray mask composed of X-ray absorber and having a pattern corresponding to a pattern of a semiconductor device to be fabricated is used. The X-ray mask is positioned in the close vicinity of a surface of a wafer on which an X-ray resist film has already been deposited. Then, X-ray is radiated onto the resist film through the X-ray mask to thereby transfer a pattern of the X-ray mask to the resist film.
The above-mentioned X-ray mask is usually comprised of a membrane through which X-ray can pass, X-ray absorbing material arranged on the membrane, a silicon substrate, and a support such as a glass plate for cooperating with the silicon substrate to support the membrane at its periphery.
FIG. 1 illustrates an example of an X-ray mask. The illustrated X-ray mask is comprised of a membrane 2 composed of silicon nitride (SiN), silicon carbide (SiC) or diamond (C) in the form of a thin film, an X-ray absorber 1 formed on the membrane 2 and having a desired pattern, a silicon substrate 3 supporting the membrane 2 at its periphery, and a support ring 4 composed of silicon carbide or quartz glass and cooperating with the silicon substrate 3 to thereby support the membrane 2.
In conventional X-ray masks, the X-ray absorber 1 has usually been composed of simple metal such as tungsten (W) and tantalum (Ta).
A method of fabricating the X-ray mask illustrated in FIG. 1 is explained hereinbelow with reference to FIGS. 2A to 2D.
First, thin films 2a are deposited on opposite surfaces of a silicon substrate 3 by chemical vapor deposition. The silicon substrate 3 has a thickness of 1 to 2 mm. The thin films 2a are composed of silicon carbide (SiC) and have a thickness of about 1 to 2 .mu.m. One of the thin films 2a will make a membrane 2.
Then, as illustrated in FIG. 2A, a support ring 4 is adhered to a lower surface of the silicon substrate 3 at its periphery by means of an adhesive such as epoxy resin. The support ring 4 is composed of glass or silicon carbide (SiC), and has a thickness of about 5 mm.
Then, as illustrated in FIG. 2B, the silicon substrate 3 is back etched from a lower surface thereof by anisotropic etching through the use of KOH aqueous solution. As a result of the etching, a portion of the silicon substrate 3 is removed, and there is formed a membrane 2 on the silicon substrate 3.
Then, as illustrated in FIG. 2C, an X-ray absorbing material 1 is deposited on the membrane 2 by sputtering.
Then, as illustrated in FIG. 2D, the X-ray absorbing material 1 is patterned into a desired pattern 1a by dry etching. Thus, there is completed an ray mask having a desired pattern 1a.
In the method illustrated in FIGS. 2A to 2D, although the silicon substrate 3 is back-etched prior to depositing and patterning the X-ray absorbing material 1, the silicon substrate 3 may be back-etched after deposition of the X-ray absorbing material 1.
In order to make a minute pattern of a semiconductor device by means of X-ray lithography, an X-ray absorbing material of which an X-ray mask is composed is required to have the following characteristics.
First, an X-ray absorbing material has to have high ability of disallowing X-ray to pass therethrough, in order to provide sufficient contrast in X-ray exposure. Herein, such ability of disallowing X-ray to pass therethrough is defined as a product of a mass absorption coefficient and a density of an X-ray absorbing material. An X-ray absorbing material is particularly required to have high ability of disallowing passage therethrough of X-ray having a wavelength of about 1 nm, which X-ray wavelength is usually used in X-ray lithography.
If an X-ray absorbing material has smaller ability of disallowing X-ray to pass therethrough, a film composed of an X-ray absorbing material, to be formed on a membrane, has to have a greater thickness. In such case, it would be quite difficult to make a minute pattern of the X-ray absorbing material.
In addition, if a film composed of an X-ray absorbing material has a great thickness, problems such as inaccurate transfer of a pattern to an X-ray absorbing material, and difficulty of controlling stress remaining in a film composed of an X-ray absorbing material, would be cause.
Second, an X-ray absorbing material has to have a stress as small as possible, and further have high controllability of stress.
If a film composed of an X-ray absorbing material, formed on a membrane has a great internal stress, positional accuracy with which a pattern is transferred to an X-ray absorbing material from an X-ray mask would be deteriorated, and as a result, misalignment would be caused in a semiconductor device pattern. Accordingly, an X-ray absorbing material is required to have almost zero internal stress all over a surface of an X-ray mask.
In addition, taking productivity of an X-ray mask into consideration, an ray mask is required to not only have an almost zero internal stress, but also be able to be repeatedly fabricated in the same configuration. Furthermore, since an X-ray mask is repeatedly employed, an X-ray mask is required to have stability in stress.
Third, an X-ray absorbing material is required to have a densified crystal structure.
Most metals having been used as an X-ray absorbing material is changed into a polycrystalline film having a columnar structure when deposited into a film by sputtering. If such a polycrystalline film is patterned, grain boundary would appear at a sidewall thereof, resulting in a side surface of a pattern having much roughness, in which case, it is no longer possible to form a desired pattern of a semiconductor device.
Apart from the above-mentioned characteristics, an X-ray absorbing material is required to have conformity to dry-etching to be carried out in patterning, and to have chemical stability.
However, conventional X-ray absorbing materials used so far cannot meet all of the above-mentioned requirements.
Tungsten (W) and tantalum (Ta) have been conventionally and widely used as an X-ray absorbing material, because these metals meet the above-mentioned first requirement. That is, these metals have sufficient ability of disallowing passage of X-ray therethrough. However, tungsten (W) and tantalum (Ta) cannot meet the above-mentioned second and third requirements, and hence, these metals cannot be used for an X-ray mask when a minute pattern of a semiconductor device is to be formed.
If a film is formed of tungsten (W) or tantalum (Ta) by sputtering, a resultant film would be a polycrystalline film having a columnar structure. Hence, when a minute pattern is to be formed of tungsten (W) or tantalum (Ta) by sputtering, grain boundary would be generated at a sidewall of a pattern to thereby rough the sidewall, which is a big hindrance to the accomplishment of a desired minute pattern.
In addition, a stress in a tungsten or tantalum film is very much dependent on conditions for forming a film by sputtering, specifically, a pressure at which a film is formed, namely, a pressure of sputtering gas, and a temperature at which a film is formed. For instance, as illustrated in FIG. 4, a stress of a tantalum film varies from a compressive stress to a tensile stress depending on a pressure of sputtering gas.
It is a key that a stress is significantly varied in a range where a stress is equal to about zero. That is, even if a film is formed in such a condition that a resultant film has a zero stress, a stress would be varied much by slight fluctuation in film-forming conditions, resulting in much deterioration in reproducibility of a film.
In addition, since a film stress is dependent on a film temperature in fabrication of a tungsten or tantalum film by sputtering, a problem of non-uniformity in a stress would be caused, if there is a difference in temperature in an X-ray mask between a central portion and a peripheral portion at which the X-ray mask is supported with a support ring.
In order to overcome such a problem, it would be necessary to control a temperature of a membrane by filling a lower surface of a membrane with helium (He), independently control a temperature of a support ring and a membrane, or strictly control a temperature of a vacuum chamber of a film deposition apparatus. As mentioned above, a film composed of tungsten or tantalum is accompanied with a problem that it is quite difficult to keep a stress uniform in a film.
Among the above-mentioned problems of a tungsten or tantalum film, various materials have been suggested in order to solve the problem with respect to crystalline structure.
For instance, as an X-ray absorbing material of which an X-ray mask is composed, M. Sugihara et al. have suggested the use of Ta.sub.4 B in Journal of Vacuum Science and Technology, Vol. B7, No. 6, 1989, pp. 1561, and H. Yabe et al. have suggested the use of WTiN in Japanese Journal of Applied Physics, Vol. 31, 1990, pp. 4210. Since Ta.sub.4 B and WTiN are both amorphous, the use of these materials can solve the problem that a sidewall of a pattern is roughed due to the above-mentioned polycrystalline structure.
However, boron (B), titanium (Ti), and nitrogen (N), which have to mixed with tantalum or tungsten, are atoms having small ability to disallow X-ray to pass therethrough. Hence, if an X-ray mask is composed of an alloy composed of tantalum or tungsten together with boron (B), titanium (Ti), or nitrogen (N), a new problem that the X-ray mask cannot avoid having a greater thickness than a thickness of an X-ray mask composed singly of tungsten (W) or tantalum (Ta) in order to provide sufficient contrast in X-ray exposure would be cause.
For instance, Japanese Unexamined Patent Publication No. 2-2109 has suggested the use of Ta and one of Al, Ti, Si and Mo for composing an X-ray mask thereof.
However, an alloy composed of Ta and one of Al, Ti and Si is accompanied with a problem of small ability of disallowing X-ray to pass therethrough. As to an alloy composed of Ta and Mo, it has no improvement in crystalline structure, since a film composed of such an alloy has a columnar polycrystalline structure.
As mentioned so far, an X-ray absorbing material of which an X-ray mask is composed, having been suggested so far, cannot meet the above-mentioned characteristics required for an X-ray absorbing material.
Apart from the above-mentioned X-ray absorbing materials, various materials have been suggested.
Japanese Unexamined Patent Publication No. 2-94421 has suggested an X-ray mask composed of tungsten (W) and rhenium (Re).
Japanese Unexamined Patent Publication No. 3-34414 has suggested an X-ray mask composed of tantalum (Ta) and nickel (Ni), a ratio of the number of atoms between tantalum and nickel being 3.0:7.0 to 5.0:5.0.
Japanese Unexamined Patent Publication No. 9-190958 suggested an X-ray mask composed of tantalum (Ta) and germanium (Ge).
However, the materials as suggested in the above-mentioned Publications cannot meet the above-mentioned characteristics required for an X-ray absorbing material.